Estradiol (E2) in adult female rodents regulates body weight, adiposity, energy balance, physical activity and glucose homeostasis, as demonstrated by robust metabolic responses to ovariectomy, E2 replacement, and genetic manipulation of estrogen receptors. In female primates, however, removal of the ovaries does not reliably lead to increases in adiposity, insulin resistance or altered energy homeostasis. In recent studies, we have confirmed that neither ablation of peripheral E2 production nor E2 replacement alters adiposity, glucoregulation or energy homeostasis in marmoset monkeys, adding to previous evidence suggesting peripheral estrogens do not play important roles in female primate metabolic regulation. Our recent novel findings, however, demonstrate the critical importance of hypothalamic estrogen receptor alpha (ER?) in regulating metabolic function in a female nonhuman primate (NHP), as silencing of hypothalamic ER? gene expression induces obesity and insulin resistance in adult female rhesus monkeys. Taken together, these findings suggest that hypothalamic ERa may function in NHPs, as it does in rodents, to regulate adiposity, glucoregulation and energy homeostasis, yet peripherally produced E2 has diminished importance in engaging these actions. Recent studies have suggested that neurosteroidogenesis, specifically hypothalamic aromatization by CYP19A1 of androgen precursors to E2, may regulate neural control of metabolic function in NHPs. We have therefore formulated a new hypothesis that E2 synthesized in the brain, specifically in neurons of the ventromedial nucleus (VMN) and arcuate nucleus (ARC) of the hypothalamus, activates ER? to regulate adiposity, glucoregulation and energy homeostasis in female NHPs. To test this hypothesis, we will use both pharmacological and viral vector- mediated shRNA approaches to determine if adiposity, glucoregulation and energy metabolism are altered by inhibition of the CYP19A1 aromatase enzyme, or by permanent silencing of the CYP19A1 gene in the hypothalamus of female rhesus macaques. We will also analyze the synthesis of E2 in the VMN and ARC by a microdialysis approach, and determine whether VMN and ARC E2 originates from hypothalamically synthesized androgens. These studies may fundamentally change our understanding of metabolic control of adiposity, glucoregulation and energy homeostasis by sex steroids in female NHPs, and prompt exploration of new therapeutic strategies to diminish metabolic disease in women.